Serious Sam is a new style FPS (first person shooter) that is very benchmark friendly. The Dunes demo is an open field full of bad guys that is more of a performance map, while the suburbs demo has many more objects, houses, trees, and enemies (much more stressful).

The KK266Plus is just able to edge out the KK266-R. Seems like Serious Sam can't really take advantage of the bus speed increase like Quake 3 did. Still, you wouldn't be able to notice the difference between the 2 systems.

MDK2 is an aging OpenGL, 3rd person shooter that has many FPS characteristics. It's use is a little outdated, however many hardware sites still use it to compare hardware. When looking at the latest hardware it provides a good indication on how things have advanced.

MDK2 like Serious Sam really can't differentiate between the 2 systems. Both are extremely fast, and it's impossible for a person to differentiate between the slowest to the fastest.